(1) Field of the Invention
The present invention relates to a sintered body comprising alumina as the main component and zirconia, and a process for the preparation thereof. More particularly, the present invention relates to an improvement of a sintered body valuable as a cutting tool, a high-temperature material, an ordinary industrial machine material or a living body material.
(2) Description of the Related Art
A tool composed of a ceramic material is advantageous in that the tool is excellent in the hardness, abrasion resistance and heat resistance, but the tool is defective in that chipping or flawing is easily caused and therefore, use or finish processing is restricted. With recent development of machine tools, the necessity to enhance the machining speed and prolong the cycle time for exchange of tools increases, and ceramic tools having a high stability and a high strength, which are capable of meeting this requirement, are desired.
Alumina (Al.sub.2 O.sub.3) is low in the reactivity with metals and is excellent in the abrasion resistance, and therefore, alumina attracts attention as the material valuable for machining tools. However, alumina involves a problem of a low breaking toughness (K.sub.1 c). Zirconia (ZrO.sub.2) has a high flexural strength and a high breaking toughness, but at 200.degree. to 300.degree. C., the strength abruptly decreases and zirconia is thermally unstable. Moreover, zirconia reacts violently with iron and a machining tool formed of zirconia cannot be practically used.
Accordingly, there has been adopted a method in which the breaking toughness of Al.sub.2 O.sub.3 is improved by incorporating and dispersing ZrO.sub.2 in Al.sub.2 O.sub.3. Two types of this improving method have been proposed. According to one proposal, monoclinic ZrO.sub.2 is dispersed in an Al.sub.2 O.sub.3 sintered body and micro-cracks are formed by the phase transition of ZrO.sub.2. According to the other proposal, a tetragonal crystal is dispersed in an Al.sub.2 O.sub.3 sintered body and the energy on the top end of the crack is absorbed by the phase transition of ZrO.sub.2.
In the above-mentioned Al.sub.2 O.sub.3 -ZrO.sub.2 type sintered body, it is known that the particle size of ZrO.sub.2 has influences on the breaking toughness and flexural strength of the sintered body. More specifically, if the particle size of dispersed ZrO.sub.2 is smaller than 1 .mu.m, the above-mentioned toughness-improving effect tends to decrease. The reason is that expansion of the volume of ZrO.sub.2 particles is inhibited by Al.sub.2 O.sub.3 particles and the phase transition of from the tetragonal crystal to the monoclinic crystal is hardly caused. On the other hand, from the viewpoint of the flexural strength, it is preferred that ZrO.sub.2 be present in the form of fine particles, and if the large ZrO.sub.2 particles having a size of 2 to 3 .mu.m are present, these particles act as the breaking source.
As is apparent from the foregoing description, in the conventional Al.sub.2 O.sub.3 -ZrO.sub.2 type sintered body, the breaking toughness and the flexural strength are not compatible with each other. Therefore, the conventional technique is defective in that a sintered body which is excellent in both of the breaking toughness and the flexural strength cannot be obtained.
More specifically, according to the former method, the breaking toughness is improved, but the flexural strength is low and the sintered body is not practically used. According to the latter method, though the effect of improving the breaking toughness is relatively lower than the effect attained according to the former method, the flexural strength is highly improved because of absorption of the energy on the top end of the crack or by the compressive stress generated by processing of the surface. However, in order to make ZrO.sub.2 present in the form of a tetragonal crystal in Al.sub.2 O.sub.3, it is necessary to disperse ZrO.sub.2 in the form of fine particles having a size of 0.3 to 0.5 .mu.m or a smaller size.
Since it is technically very difficult to control the particle size of ZrO.sub.2 to less than 0.5 .mu.m in the sintered body, there has generally been adopted a method in which a small amount of a stabilizer such as MgO, CaO or Y.sub.2 O.sub.3 is added so that metastable tetragonal ZrO.sub.2 is formed even if the particle size of ZrO.sub.2 particles is about 1 .mu.m.
This method is insufficient in that because of non-uniform dispersion of the additive, some particles are excessively stabilized and do not participate in absorption of the energy on the top end of the crack, but metastabilization of tetragonal crystals cannot be attained without addition of the additive, and it is substantially impossible to metastabilize cubic crystals without addition of the additive.